Recently, an electrically driven vehicle that is mounted with a power storage apparatus (for example, a secondary battery and a capacitor), and travels by using driving force generated from electric power stored in the power storage apparatus is attracting attention as an environmentally-friendly vehicle. Examples of such an electrically driven vehicle include an electric automobile, a hybrid automobile, a fuel-cell vehicle, and the like.
These electrically driven vehicles each sometimes include a motor generator for receiving electric power from the power storage apparatus to generate driving force for traveling at the time of starting or acceleration, and generating power by regenerative braking at the time of braking to store electric energy in the power storage apparatus. Thus, the electrically driven vehicle is mounted with an inverter to control a motor generator in accordance with a traveling state.
Such a vehicle sometimes includes a voltage conversion apparatus (converter) between the power storage apparatus and the inverter to stably supply electric power utilized by the inverter that varies depending on a vehicle state. The input voltage of the inverter is made higher than the output voltage of the power storage apparatus by this converter, thereby enabling high output of a motor. Additionally, a motor current is reduced at the time of the same output, thereby enabling downsizing of the inverter and the motor, and reduction in cost.
In order to further improve the fuel efficiency of the electrically driven vehicle, it is important to reduce the loss of the converter to improve efficiency.
For example, in Patent Document 1, there is proposed a technology of switching a step-up converter by one arm in an area where load power is small. According to such a technology, it is possible to reduce the loss of the converter by a reduced current ripple.
Additionally, for example, in Patent Document 2, there is proposed a technology of fixing an auxiliary switching element and a main switching element to be in an ON-state with one arm, and then turning on or off a main switching element on the other side according to a duty ratio, in a case where the duty ratio becomes higher than a predetermined value.    Patent Document 1: WO 2010/137127    Patent Document 2: Japanese Patent Application Publication No. 2010-283931